Light entering the eye through the cornea is focussed through the lens (which further focuses the light) onto the retina, a thin layer of cells in the back of the eye. Normal human vision depends upon signals generated by nerve cells in the retina. The visual signals originate with the photoreceptor cells in the retina, which sense and respond to light, generating signals that in turn create and shape nerve signals in retinal ganglion cells. Nerve cells often have extended cellular portions called cell processes, which may be specialized for receiving information and stimulation, or for transmitting information. For example, the specialized elongated processes that conduct nerve impulses are termed axons. The axons of the retinal ganglion cells carry the visual signals from the retina to the brain. In the brain, nerve cell networks process the visual signals further to provide the full visual experience of a normally-sighted person. Disturbances at any step in the process may lead to visual impairment or blindness.
Age-related macular degeneration (AMD) is one of the most common forms of blindness in people over the age of 65. Currently, there is no effective treatment for most patients with AMD, a disease that often results in permanent damage to photoreceptors, but spares most retinal ganglion cells (RGCs). Similarly, other diseases such as retinitis pigmentosa (RP) cause vision impairment and blindness due to loss of photoreceptors.
Inherent to the power of the human visual system is the ability to transduce light by individual photoreceptors, thus making it a high-resolution image capture system. Several groups worldwide have carried out clinical experiments to determine if stimulating retinal cells, the optic nerve bundle, or cells of the visual cortex with microelectrode arrays can generate phosphenes (i.e., sensations of light) in individuals blinded from AMD. The electrical fields produced by the microelectrode arrays stimulate relatively large regions containing numerous neuronal and glial cells. These trials have shown that by stimulating neurons with a microelectrode array, blind individuals can indeed recognize a simple pattern such as a horizontal or vertical line. Although these trials have demonstrated that vision is recoverable in a limited fashion, major challenges remain. Due to the size and difficulties in placement of most available electrodes, imprecise electric field stimulation extending over long distances (several cell-body diameters) is used to depolarize neurons. However, such methods often require excessive stimulation, which may be harmful, leading to inflammation of the stimulated region and even to excessive growth of glial cells, or gliosis. Thus, an unmet major challenge of these approaches is that of constructing a neural interface that stimulates localized retinal regions, individual neurons, and even specialized portions of neurons with specificity.
Neurons may be grown on artificial substrates. However, the synaptic connections of neurons grown on artificial substrates may not be controlled or precisely directed to defined locations, and do not provide for the specific stimulation characteristics found in vivo.
Accordingly, methods and devices are needed that improve the specificity of neural stimulation, and preferably improve the specificity of neural stimulation with low power delivery to avoid gliosis and inflammation.